With the development of communications system, various modulation modes of high frequency spectrum efficiency are widely used. These modulation signals have unsteady envelopes that put very high demands on linearity of the power amplifier inside the transmitter. The method of linearization therefore becomes an indispensable technique. Due to its simplicity, flexibility and low cost, digital baseband predistortion has become an effective method of power amplification linearization.
Predistortion method employing scalar information (such as the out-band power and in-band/out-band power ratios of a power-amplified output signal) as the optimization objective can avoid influences of such factors as the delay of the feedback loop and I/O unbalancing, and is more simple and effective.
FIG. 1 is a block diagram illustrating the principles of a power amplification apparatus employing a prior art method. As shown in FIG. 1, after baseband information source information x(n) from an information source 100 passes through a lookup table predistorter 102, an amplitude predistortion value yam(|x(n)|) and a phase predistortion value ypm(|x(n)|) are obtained, and a multiplier 103 multiples the baseband information source information with the predistortion values to obtain a predistorted signal x(n)·yam(|x(n)|)·exp(j·ypm(|x(n)|)), which is converted into an analogue signal by a digital-to-analogue (D/A) converter 104. This signal is up-converted by an up-converter 105 into an RF signal and then inputted into a power amplifier 106. The signal amplified by the power amplifier 106 is transmitted by an antenna. At the same time, partial signal outputted from power amplification is down-converted by a down-converter 107 to be fed back to an analogue-to-digital converter 108, which samples the digital signal to obtain a fed-back digital baseband signal of the output of the power amplifier. The baseband signal is analyzed by digital signal processing technology in an out-band power calculating module 109 to obtain the out-band power value of this digital baseband signal. Subsequently in a module 110, the out-band power value is optimized as a target function, and parameters of the predistorter are updated by using an optimization algorithm, so as to realize an adaptive predistortion process.
One essential problem in the algorithm rests in using what kind of structure and algorithm to realize the lookup table predistorter as well as update of the predistortion lookup tables used therein.
U.S. Pat. No. 6,600,792 selects predistortion values at several amplitude points as predistortion parameters. These parameters are discrete points on a predistortion curve. These discrete points are updated point by point during the process of parameter update, and a predistortion lookup table is entirely generated by the method of linear interpolation or spline interpolation. With regard to this method, when there are many items in the lookup table, interpolation operation leads to huge computational amount, which is difficult for real-time implementation. U.S. Pat. No. 6,731,168 prestores two lookup tables one under high temperature condition and another under low temperature condition, adaptively controls interpolation factors in accordance with feedback temperature information, and performs interpolation between the two tables to obtain the current predistortion table. Although this method requires few predistortion parameters, the predistortion table obtained thereby has relatively low degree of freedom and inferior flexibility. U.S. Pat. No. 6,240,278 makes use of orthogonal polynomials as predistortion functions, and achieves adaptability by updating coefficients of the orthogonal polynomials. The orthogonal polynomials required in this method depend on statistical characteristics of the input signals, and corresponding orthogonal polynomials should be calculated anew under different characteristics of the input signals. There are some other algorithms that calculate a set of predistortion parameters by measuring power amplification characteristics under various environmental conditions, and select corresponding predistortion parameters according to the current operating status. These methods also have inferior flexibility while requiring more storage capacity.